The present invention relates generally to a control system and a control method as well as to a control state judgment device and a control state judgment method. More particularly, the invention pertains to a system and a method which can offer an increased accuracy in detecting an oscillating state in feedback control operation.
A situation occasionally experienced in feedback control operation is oscillation of a controlled system, or subject, in which a deviation of a quantity to be controlled from a target value thereof cyclically increases and decreases in a regular pattern. It is desirable to instantly detect such an oscillating state and vary appropriate control parameters in a manner suitable for the situation.
When the controlled subject is subjected to cyclically occurring external disturbances, the deviation of the controlled quantity from the target value oscillates (repetitively increases and decreases) in a regular pattern. In the presence of such oscillation, it is difficult to distinguish between the oscillation caused by the disturbances and ordinary oscillation caused by the feedback control operation. In a process of controlling the heading of a ship, for example, it is difficult to distinguish between an oscillating state caused by feedback control operation and an oscillating state caused by the influence of such external disturbances as cyclical rises and falls of ocean waves. Although it might be possible to distinguish between two kinds of oscillation based on whether the amplitude of periodic changes in the controlled quantity exceeds a preset threshold or not, the value of the threshold to be preset greatly varies with the type and nature of the controlled subject and seriousness of disturbances, so that this threshold approach imposes a great deal of difficulty in designing a control system.